Blow Moulding Tools

Blow moulding, also known as hollow moulding, is a manufacturing process for forming hollow plastic parts. In general, there are three main types of blow moulding: extrusion blow moulding, injection blow moulding, and injection stretch blow moulding. The blow moulding process begins by melting the plastic and forming it into a form blank, or in the case of injection and injection stretch blow (ISB) preformed parts. The blanks are tubular plastic parts with a hole at one end through which compressed air can pass. It is a moulding method that uses gas pressure to blow the thermoplastic closed in the blow moulding tools to form hollow products.

The mold blank is then clamped into the mold and air is blown into it. The air pressure is then pushed out of the plastic to match the mold. Once the plastic has cooled and hardened, the mold is opened and the part is ejected.

Compared with injection moulding, extrusion blow moulding has the following advantages.

(1) blow moulding machinery (especially blow moulding molds) is less expensive (moulding similar products, the cost of blow moulding machinery for injection moulding machinery about 1 / 3-1 / 2), the production cost of products is also lower.

(2) blow moulding, the billet is formed under low pressure through the head and blowing at low pressure (mostly 0.2-1.0MN), so the residual stress of the product is smaller, higher resistance to tensile, impact, bending and environmental strains and other properties, with better performance. In injection moulding, the melt has to pass through the mold runners and gates under high pressure (15-140yPa), which leads to uneven stress distribution.

(3) The relative molecular mass of blow moulding grade plastics (e.g. PE) is much higher than that of injection grade plastics. Therefore, blow molded products have high impact toughness and high resistance to environmental stress cracking, making them suitable for the production of containers or vats for packaging or transporting detergents and chemical reagents.

(4) Because the blow moulding die consists only of a negative die, the wall thickness of the product can be changed by simply adjusting the head die gap or extrusion conditions, which is advantageous for products that cannot be accurately calculated in advance for the required wall thickness. For injection moulding, the cost of changing the wall thickness of the product is much higher.

(5) Blow moulding can produce products with very small wall thickness, which cannot be produced by injection moulding.

(6) Blow moulding can produce products with complex, irregular and monolithic shapes. When injection moulding is used, two or more pieces of products should be produced and then combined together by latching, solvent teaching or ultrasonic welding.

Common Defects

1 not adequate finish for preform mold causes scratches when pulling or material stuck on screw after carbonized;

2 wrong blow direction or much higher pressure causes blow up;

3 too slow blowing speed makes material deposit in part;

4 uneven wall thickness gets uneven stress during cooling which causes warp;

5 part weight not correct when too long blow time or too much pressure;

Notice for Part Design

As making blow moulding tools for vehicle parts like spoilers, air ducts, tanks,etc. It is not easy to control product wall thickness and when design, better to make simpler structure and proper parting line, normally 0.8mm ~1.5mm.

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